Cost-Effectiveness of Inline, Offline, and Nested Stormwater Control Measures in an Ultra- Urban Watershed
Abstract
Tefera Shibeshi* and Temesgen Mekuriaw Manderso
Urban watersheds characterized by extensive impervious surfaces and combined sewer systems face ongoing challenges related to excessive runoff, pollutant loading, and flooding. Despite the widespread implementation of green and gray stormwater control measures (SCMs), empirical guidance on the relative cost-effectiveness of different SCM configurations in space-constrained urban settings is limited. This study conducted a performance-based modeling assessment to compare inline, offline, and nested SCM strategies within a 471-acre ultra-urban sub-watershed draining into the tidal Delaware River in Philadelphia, USA. Utilizing the python script(pysewer) in conjunction with a scenario- based optimization framework, various SCM configurations were evaluated based on annual runoff reduction, annual zinc (Zn) load reduction, and estimated implementation cost. The results indicated that a strategically located inline bioretention system achieved the highest cost-effectiveness, providing approximately 180 lb yr−1 of zinc reduction at minimal cost, whereas offline systems demonstrated moderate performance at significantly higher costs. Nested SCM configurations exhibited the lowest cost-effectiveness owing to hydraulic redundancy and inefficient flow diversion, despite the higher total investment. These findings demonstrate that simpler, well-sized SCMs can outperform more complex treatment trains in ultra- urban environments and underscore the importance of performance-based design for stormwater planning under constrained conditions in ultra-urban environments.